I think "必定" is just the same as "一定". It's just that people don't use "必定" that much now. So I think "一定" is better.

我认为用“一定”比较好。 “必定”的语气要比“一定”更强，对于外国人来说，虽然学习太极拳会有困难，但不见得就是“必定”，这和一个人的运动能力有关，有些外国人学起中国的武术来，并不比中国人差。如果说针对一个特定的外国人来说，可能他不擅长运动，确实会有困难，说成一定也就够了。

yvonne's answer is really a standard & correct answer! Actually in normal life, we don't distinguish such a subtle difference. We just feel that "必定" is very very intensive affirmative, and "一定" is not so strong, just a normal affirmative speculation. But if you really want to do this, you must know that the key to distinguish similar chinese words is to focus on their different part. In this question, their different part is "必"、"一". And as yvonne's answer said, "必定" suggests that there is some irresistible factor, so it's an objective speculation, maybe that's why this words sounds very very affirmative.

1、在意思上：“必定”就是“一定”的意思，都表示必然的、肯定的判断。它们是同义词。2、差异在于：“必定”所表示的“必然的结果”，更多因为某些客观因素决定，而不因个人意志改变。 所以这个问题的答案就应该清楚了：客观情况是“他是一位外国人”，后面填空应该用“必定”。

我觉得两个可以，如果一定要选一个，用“必定”吧，可以表示判断猜测。